All of a sudden I cannot enter the strip! I need a key? Please tell me what I did to trigger this and how to fix the problem.
A lot of my quests need me to go there.

There are only two way to get into the strip: 1.) have a parssport a NPC has provided (I haven't found out who yet). 2.) pass a credit check of having 2,000 caps in hand. If you got in before and cannot get in now, it probably means your caps dropped lower then 2,000. Shot some more things and sell them.

Many people get this around level 16. If you find a way to quickly and quietly kill one or all of the securitrons, they will have a key on them for the door. This isn't proven to work for everyone but has been a solution in the past.

If you're on the PC version:

Open the console. (~ key)
Click on the door so you see it's ID at the top.
Type: unlock

Carry on.

Sadly the PS3/360 do not have a console so they are SOL in this regard.

Happened to me earlier today. What i done is went to Camp McCarran and took the monorail into the Strip, then the gate unlocked by itself. Hope it works out alright. By the way, You might need wear some NCR armor to use the monorail and sneak in

I've never had to worry about a credit check once I passed the initial one, and have walked right in with only 16 caps to my name before.

I stand corrected. That was how I THOUGHT it worked. :facepalm: