TI-84 Silver Edition help?

I've had my calculator for about 3 years now and it's worked perfectly. The problem is graphing. Last year it was perfect, then this year I tried using it to graph and it will show the graph for three seconds and then go to ERR: SYNTAX. My professor messed with it and everything was right, the window was in the correct degrees but it won't hold the graph so I can do anything with it. It computes perfectly other than showing the graph. Now, this wouldn't be an issue if I didn't need the graphing part but next semester I plan on going into Pre-Calc and was told I needed that part of my calculator. Can anyone help me on how to fix my calculator? I'd really like to not spend another 100 dollars on a new calculator when I'm using it for only one more semester and not taking another math course. Thanks.