If you cannot find your DOT address on a block explorer, it has either not received the minimum amount of DOT required to activate the account yet, or its balance dropped below that amount and it was reaped (deactivated).

  

This amount is called the existential deposit (ED) and is currently set at 0.0000333333 KSM for the Kusama network and 1 DOT for Polkadot. 


You can (re-)activate a reaped account by sending at least the ED to it. This will make it findable on block explorers again.


NOTE: Most block explorers keep the transaction history of reaped accounts, so you can still find them even if they've been deactivated.