In order to access your DOT account on your Ledger, click "Add Ledger" on the polkadot.js accounts page.

On the next screen, make sure to enter the exact same Account Type and Index as you did the first time:

This is important because every combination you choose here will produce a different DOT address. So in order to access your address, you need to select the correct Account Type and Index here. 

Most people just leave it at the default of 0 and 0. Maybe you selected 1/0 or 1/1 or 0/1 when you first connected your Ledger. If you do not remember, then you have to try out several combinations until you find your address. If you already sent funds to it you will immediately recognise it as you will see your DOT balance.

Once you found it, remember the Account Type/Index combination, as you will always have to select it in order to access this specific address. 

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