Some follow up questions.
how does your osTicket make tickets?
What I mean by that:
Do customers go to the web UI and open them?
If so then you should probably check and make sure that it is working.
Do customers usually send an email to a specific address that makes tickets?
If yes, then how does osTicket get those emails to make the ticket?
(There are a couple ways: Piping or checking the mailbox)
Try sending an email to the address. Does the mailbox receive the email?
If not then the problem is likely with the mail account.
If yes, then we need to know if your using Piping or checking the mailbox (IMAP/POP/etc).
If Piping then I would think that there would be an error in a log somewhere.
If logging into the account and collecting emails then there should be a log for that access.
Also if using IMAP/POP then you are likely using cron every few minutes-ish to run the process that does the collection. You would want to check that it's still running.
