Yeah, you're working on four weeks with blocked sinuses... if it lasts much longer it's time for a trip to the doc. Most colds don't last any longer than 4-6 weeks, and along those lines, full sinus blockage for that kind of extended period of time is less common.
As to whether or not you think you can get back to the gym, this is really up to you, but there are a couple things to consider. If you're still having the occasional fever, you are very likely still contagious, and it's not fair to other gym-goers to expose them to your illness. Outside of that, it's really up to you, and how well you feel. You should weigh any soreness or weakness caused by your cold against your workout, but also remember that physical exhaustion can work against you in recovery.