Hello Sagar,
There is a module in the learning area of the portal about getting the kind of information required to help with an issue.
My guess is that you have not built your process based upon a process template - which would have best practice exception handling in. If best practice exception handling is incorporated what your process should be doing when this very occassional issue occurs is catching the error, tidying up the application (which may involve restart it), renavigating to where it needs to be, and trying again. It will do this retry loop a few times before letting the exception bubble up to your main page.
This retry exception handling logic tends to fix occassional system errors like what you describe - for more details there is an Exception Handling guide on the portal, instructions distributed with the Process Template, and an example process in the Lifecycle Orientation training.