@Richard's response is safest, but ... check something before you begin ...
Strange that all links to assignments would disappear like that ... bet the data is still in the DB (for your sake, am hoping).
Before the upgrade took place, did you do a full site backup? ... that included a DB dump.
Before doing the following, strongly suggest making a DB dump and make sure the name of the dump indicates in it's name something like '...p2upassign...' (prior 2 upgrading assignments) and the Moodle's current version (so we don't get confused when/if having to use those dumps to restore to a 'state').
Is there an 'upgrade assignments' link in the site admin menu? I have gained admin levels to sites that did not convert old assignments to new assignments back when the change took place ... around version 2.3.x or so ... and had to convert old assignments to new.
The menu link will provide a page that has a link to list all assignments that need converting. Click it. It will present links to old assignments ... something like 20 at a time with page links (in your case a bunch of them). Should have a drop down to select the number of assignments to convert at one time. Suggest you might have to set to the lowest (not highest) and step through many similar pages to get them all converted. Had one site that literally took 3 hours ... lots of clicking on next page of assignments to be converted .. 10 at a time. Anything else, however, would fail ... just too much processing for server to do.
Oh, yeah, might be a good idea to put the site in the maintenance mode while you are doing the above - however long it takes.
Once all assignments have been converted, go to the modules and hide the old assignment mod leaving only the new assignment mod to be used when/if new assignments are created.
'spirit of sharing', Ken