Do you have Train-store? I am not sure how much stuff you have on you set up but there comes a point when MSTS can't handle it all and gives up. Train-Store is one way to work around that.
You can also use Route-Riter or Conbuilder to check the activity. You are probably missing something the activity needs to work. Some activities are very needy of downloads. Both of these tools will let you replace items you don't have with ones you do.
Reinstalling may not solve your problem. Often MSTS just rewrites to the old spot, keeping all the add-ons (Don't ask how I know this).
It sounds like the problem is a missing piece of rolling stock or locomotive. That should be easy to fix. Double check the documents included with the activity and be sure you have all the required pieces.
If it was something you added, you have several choices. Either use Conbuilder (used to be free, now there is a lite version which is free and a payware full version) to check the activities in the route. It will list all the missing or defective rolling stock. You can just add something you have for it in the consist and save it.
You can use Route Riter to do basically the same thing. It has a different tool which will check the route and the activities and make sure what you have will work. There is one tool which identifies all the problems from the route (or all of them) and lets you fix them by substituting rolling stock and locomotives. It also has a tool to fix consists and activities so they match. Sometimes people use a capital letter or something in the name while the next guy has it as a lower case letter, which confuses MSTS.
Or you can use Train Store to store all the routes and then just load what you want to drive that time. It is a way to isolate the problem activity by not loading it. This tool also has the benefit of allowing you to move the start time for activities, and of running explore route trains with traffic.
Hope this helps.