Question

When using the 'Run an Apex Class' action, I get an error message stating 'Apex Class invalid'.

Answer

Recompiling your classes will solve this:

1. Go to Salesforce Setup > Apex Classes
2. At the Apex Classes main page, click Compile All Classes
   

To prevent this from happening again, enable compiling upon deploy:

1. Go to Salesforce Setup > Apex Settings
   
2. Enable Perform Synchronous Compile on Deploy
3. Click Save