Do you have an OSGI bundle that has the servlet for your form defined? Or are you using the OOB CQ form?
If you are using the OOB CQ form, what action are you trying to do?
If you have an OSGI bundle for the Servlet logic, go to http://localhost:4502/system/console and view the bundles associated with your CQ instance listed there.
Find your OSGI bundle in the list and see if it is running (running bundles will have a 'stop' button beside them). If it is not running, try force starting it. If that does not work, there must be some issues with how you had built the bundle (Issues with your maven project if you use one to build the OSGI bundle).