I spent at least 4 hours today to complete the “Oh, it’s on!” and “Forerunner Fragfest” challenges. But after I finished the level from beginning to end, it didn’t give me any of the XP points (which I believe is 40,000 altogether). Is there a way I can get these XP points? Because there is no way I’m wasting that many hours on nothing.
According to your service record, you did not complete the challenge.
My suggestion is to double check you have every requirement for the challenge activated. This has happened to me on more than one occasion before, where I forgot to put a skull on.
I’m willing to bet you forgot to turn on one or both of Tough Luck or Thunderstorm. I’ve done the same thing PLENTY of times. 
You have gold right? If you don’t have gold, the challenges don’t work. You could have also forgotten some of the requirements, too. Also, you might have hit the xp cap for that day, which is 150xp BTW.
Yes, I have Gold and I did check to make sure I had every skull on because I knew how long it would take and I didn’t want to do it again. So I checked three times before I started it. And that’s all I did that day.
I think the problem was that it disconnected me from Live at some point without telling me and I didn’t find out until after I had finished the whole level. So if nothing else, I guess I was wondering if there was a way to contact someone to have that XP count. Otherwise it was a complete waste of my whole evening.
probably.