Check:
- 1 of your public addresses, see if you balance is still there :X
- URL (so many Blockchain phishing sites, although LastPass should not work on phishing sites)
- Alias / Identifier (sometimes you have to confirm the device thru mail with alias)
- Connection (I am pretty sure Blockchain can be strange when the connection is not so good)
- 2FA (I obviously assume you do not have 2FA? But if you do, check it. If you don't have > add next time)
- History of passwords (LastPass saves previous used passwords, maybe you have some Blockchain account mixed up)
- If nothing works, check for a backup (most times they send it thru mail)
What error (if any) do you get anyway?