If a validator you’re delegated to drops out of the Top 100, it automatically gets removed from the validator set.
This means that the validator node won’t be making proposals and thus won’t earn any staking rewards, until it reaches back into the Top 100.
It is advised in those cases to either contact the node runner (to see if he/she has any plans to push the validator back into the top 100) or to request an unstake and join another validator in the top 100.
Keep in mind that unstaking always takes 500 epochs (read more about why here.)