You can try calling the lender or logging on to your account online to get the 1098 info. You don't need the physical form if you're filing electronically.
If you can't get that info, then you can file for an extension. If you don't owe any tax, then that shouldn't be a problem. If you do owe tax, then you still have to pay it by the normal deadline to avoid penalties and interest.
If you don't want to file for an extension, and you aren't itemizing your deductions, then you can just leave off the mortgage interest. If you aren't itemizing, then I don't think you have to report your mortgage interest paid.
Worst case scenario, you can file your taxes, and then file an amended return later. But I think you have to do the amended return on paper and mail it in, and you'd still have to pay what you owe by the normal deadline to avoid penalties and interest.