If the computer tells you that there isn't enough memory available to run a certain program or load a particular document, those settings can often be fixed in the finder. If there is an error code on the screen, jot it down for reference. Occasionally, the computer is unresponsive and offers you no help. It may be totally frozen and require a restart. The cursor moves, but you can't seem to attact the computer's attention.
Some applications are very CPU intensive and can slow the computers response time to a crawl.
Often the best way to handle this is to just wait until the computer has completed the operation it is performing. Sometimes this just isnt' practical. The standard way to tell a Macintosh computer to stop and listen is to hold the command key and tap the period key a few times. Often this will stop the process and return control of the computer to you. Restarting the application and the computer are not necessary.