Lost all my downloaded emails and client also now fails to sync with gmail

System.IO.IOException: The process cannot access the file because it is being used by another process.
   at System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)
   at System.IO.__Error.WinIOError()
   at System.IO.File.InternalMove(String sourceFileName, String destFileName, Boolean checkHost)
   at DbRepair.CorruptionChecker.CheckCorruption(RepairOptions repairOptions, CancellationToken token)
   at DbRepair.CheckingProcess.Check(CancellationToken token)
databaseFile: \accounts.dat

This is the same question posed here:
and here:

In all cases can you could restart Windows, then temporarily disable any antivirus, and try again.

If that does not help, and you are using IMAP, you can delete your profile folder (C:\Users\yourname\AppData\Roaming\eM Client) and start again.