Hello, I've been receiving a specific error when I turn on the N900. The message reads "All telephony functions, including emerency calls are disabled due to a communication eeror. To recover, you might have to reboot the device". I recent bought a Simple Mobile sim card and it's still showing the no sim card inserted icon.

Sounds like your sim card is not connecting properly.
WARNING: USE OF A HAMMER INCLUDED

http://talk.maemo.org/showthread.php?t=80759

Use at your own risk.

But this has been happening before I put in the sim card.

Sounds like a hardware problem that hammer 'won't fix'. Try using some cleaning agent (99% spirit) on the copper parts as maybe interference of dirt/mold/etc is making the phone think the sim is invalid when none is inserted. Without one this message is strange.
http://talk.maemo.org/showpost.php?p...4&postcount=19
or reflash.

OK, I'll try the cleaning agent. If that doesn't work, how can I reflash it?

Maybe it is your problem: this.
In last answers I wrote how to solve it.
Even if phone says "SIM Problem", it can be GSM module problem. Watch logs.