Great question as this is a very common, and stressful, situation for inventors to be in. You should review your options with a patent attorney as your options could be magnified or limited based on your specifics. Here are some general options though:
(1) Re-file another provisional patent application. This option remains intact as long as your invention is never publicly disclosed or for sale because once a public disclosure occurs, resubmission of a provisional patent application is barred. A new provisional patent application generates a new filing and priority date. Thus, you will now have to consider any possible prior art that may have been created after your previous application.
(2) If you have disclosed your invention, then improve or alter your invention and file a new patent application.
(3) File a non-provisional patent application within 12 months of the date you filed your provisional patent application.
(4) An alternative to filing a corresponding non-provisional application is to convert the provisional application to a non-provisional application by filing a grantable petition under 37 C.F.R. §1.53(c)(3) within 12 months of the provisional application filing date.
(5) Abandon your invention. Probably the least appealing option for all you inventors out there 🙂
For more information, check out the USPTO’s page on provisional patent applications.