This question has been asked many times in the Forum. If you search for "mission email", you'll find a lot of discussion. The last post in this thread:
Need help with mission email
discusses some up-to-date info, and there's other threads like this one:
Accessing missionary emails after mission
Bottom line, if you're within a certain window of time (some have suggested 18 months, others 180 days), your clerk can give you the local number for the Global Service Center, and they can support you. If you're outside that window, it seems likely you may have lost access to your missionary email account.