On Polkadot there are some limits to the number of nominators that we can have, as well as the minimum bonded DOT an account must have in order to be able to nominate. These limits can be changed through governance and are currently set at 22,500 maximum nominators and 10 DOT minimum bond.

If you are nominating with less than the minimum bond and the current number of nominators is above 90% of the maximum nominators limit, anyone can call an extrinsic called "chillOther" and chill your account, which means make your account stop nominating. So, if you suddenly stopped nominating this is probably the reason.

In that case your DOT is not automatically unbonded. It remains bonded but you're simply not nominating anyone, and if you try to nominate again while your bond is lower than the minimum, you'll get an "InsufficientBond" error. 

An account that has bonded DOT but isn't nominating will have these two buttons next to it in the Account actions page on Polkadot-JS UI:

Why do these limits exist?

These limits can indeed exclude smaller nominators from participating in the security of the network, but they are needed in order to ensure the stability of the network.

The way the Nominated Proof of Stake (NPoS) consensus mechanism works is that it uses Phragmén algorithm to map the nominators to their selected validators and produce the active set of validators based on some criteria. This is a computational intensive operation that happens off-chain by the nodes but must be evaluated on-chain within a single block, meaning in 6 seconds.

When the number of nominators becomes too large, nodes run out of memory when they try to evaluate the proposed solutions and block production halts or becomes very slow. This happened in spring 2021.

To prevent this from happening again, the limit to the maximum number of nominators was introduced as a temporary solution. The number after which the out of memory issue occurs was calculated after rigorous testing and benchmarking on common validator hardware. However, it's not a fixed number since dependant on the specific mapping of nominators to validators, that changes from era to era. So, to be on the safe side, the imposed limit is quite conservative to ensure we don't run into the same problem again.

Since not everyone can nominate, the question then arises as to who should be able to nominate. Because one of the goals of the Phragmén algorithm is to bond as much DOT as possible, the decision was made that the accounts with a higher stake should be the ones included. Again, based on the status of the chain at the time, an initial minimum bond of 40 DOT was decided. But because both of these limits would need to be re-evaluated from time to time, they were introduced as variable that can be changed through governance. Since they were introduced, and in order to keep the nominators below 22,500 the minimum bond has been changed several times to the value it is today.

What are my options now?

If you were forcibly chilled, you have a few options to be able to stake again:

1. You can bond more DOT to go above the minimum bond limit and then you'll be able to nominate again, as long as we haven't reached the maximum nominators limit.

2. You can unbond your DOT and move them to a custodial solution that offers staking. Since custodians pool together all their users' DOT, the minimum bond limit won't apply for you there, but each custodian may have their own limitations, so make sure to check for those. Unbonding your DOT takes 28 days.

3. You can stake on Kusama. Similar limits exist on Kusama as well, but because the number of nominators there is far smaller than on Polkadot they don't have any practical impact.

What is being done to solve this?

As mentioned above, these limits are a temporary solution until a more permanent one is developed. The end goal is to move all staking election functionality to a common-good parachain, but until that happens several other solutions are being worked on.

The first one that has already been implemented is the "bags list", where nominators are placed into bags based on their stake and the top bags are selected to participate. This effectively creates a dynamic minimum bond limit, that may increase over time, but it's the first step towards the multi-block election solution that will allow a much larger number of nominators to be selected.

Additionally to that, Nominator Pools are in the works and they will allow nominators to delegate their stake to a pool, which will then nominate as a single nominator with all the pooled tokens. This way many nominators who can't participate directly due to small stake will be able to participate through a nominator pool.

You can find a lot more details about these solutions and the progress being made here.