The $2 bill used to be worth two dollars, (duh) but now it's worth nothing in most places. Why?
The United States two-dollar bill is still legal tender, is still in production, and is still worth $2.
That having been said, something is worth only what someone will give you for it. Pretend that I walk into a store with a pocket full of nothing but two-dollar bills, and the clerk in the store looks at them and says, "I don't recognize this. It looks fake. You can't use that here." Those two-dollar bills are worth nothing to me at that moment. However, if I go into a store next door where the clerk is a bit more sophisticated, they are then worth $2.
You might find yourself in a situation where someone is impressed by the two-dollar bill, having not seen it in years. He might say to you, "Can I have one of those? I'll give you three singles for it." The bill is worth $3 at that moment.
Regardless, because it is legal tender, if you find yourself with two-dollar bills and you are having trouble finding a place to spend them, any bank should accept them and exchange them for a different denomination of currency. I occasionally get a small stack of two-dollar bills from my credit union, just for fun. People usually enjoy seeing them.
Presuming that this is the US, the $2 bill is still legal tender, and so is still worth $2. Source
I have a stack of them, and they are doing fine. $1000 will buy 500 of these at my bank, and I go through them over a few years.
The premise of your question can use clarification. If a $1 bill still has value, then a $2 bill has twice that value. They are still printed, and still accepted as payment except by those who are so ignorant they don't believe they are real. Last time I was in NYC, I gave these away to those asking for money. Not one person called after me thinking it was a fake bill. A few gave me an extra thanks for a "Lucky $2"
The two dollar bill used to be worth two dollars, (duh) but now it's worth nothing in most places. Why is this?
The strict answer to your question is inflation. According to https://www.bls.gov/data/inflation_calculator.htm $2 in 1976 (when that bill was reissued) is now worth $0.45 (in 1976 terms).
But, as @Magua noted, it's still legal tender. Thus, your question is quite confused.