There are a number of causes of this.
If you are opening up the same session or route again, you should choose Revert rather than use changes because whatever caused the crash will continue to cause the crash. I say this from experience because I've always had further crashes when continuing rather than reverting and starting over from the previous save.
First run a database repair to clean up any mess that has occurred from the crash. Usually, if you've had a crash opening up the same route or session will cause the crash again, so you need to run a DBR first before trying again.
How to run a Database Repair (DBR)
At the Launcher, click on Developer on the menu and choose Rebuild database.
Once this is completed, open Content Manager and then change the view to Faulty.
If you have any faulty assets, select the assets and choose View Errors and Warnings.
If there are any errors still left, change the view to all in the filter to refresh and then check the faulty assets again. If there are still errors left, you need to address them.
Now, the usual culprits of crashes are faulty assets, obsolete or bad scripts, or corrupted data caused by some other hardware or system problem.
Ensure your content is up-to-date and error free. Check for updates on the DLS in Content Manager and delete the obsolete assets after updating.
If there are faulty scripts, you will have to find a substitute unless there's an update for those.
Generally, keeping your content updated and error free is the best way to prevent crashes, although they still occur from time to time.
