You open MultiBit Classic and instead of seeing your wallet(s) in the Wallet list, you don't see anything. The wallet(s) have probably just been closed.
Relax - you are very likely to still have your wallet(s) on your disk. Do the following:
If you've checked the file locations you should consider using file recovery software for your computer. We cannot directly recommend a particular vendor since we don't have the resources to independently verify all offerings and their latest updates. A Google search for "file recovery software" should yield popular solutions.
During the recovery process you are looking for files ending in ".wallet" or ".key" (if you've done a private key export).
Note that MultiBit Classic will not delete your wallets. If you suspect foul play you should run up to date anti-virus software to ensure you have not been infected with malware.
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