Ugh, so I went and left the headlights on overnight and drained the battery. I hooked up some booster cables to jump start my car (I've done this before and have had no problems jump starting this car in the past). After hooking up the cables, I go to try to start my car but it won't turnover... when I turn the key, the dashlights etc come on (including the orange check engine light which then clicks out/off) but when I try to get it to start, it makes no noise. I can't even hear clicking.

The domelight, headlights and dashboard lights do come on now but that's it. My battery is over 5 years old and have drained it a couple of times... is it possible I just need to replace the battery? Or is it more likely a bad selenoid?

Sounds like the battery is bad, especially being 5 years old. You could try putting it on a charger. Costco has the best deal on replacements that I know of.