Seriously, though, it could be alot of things. Too many to list here. Take the tube out, pump it up, and find where the leak is, and if you find the leak and can't figure out what went wrong, post your findings here and see if anyone can help. Tubes don't typically just spontaneously die.
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"When once you have tasted flight, you will forever walk the earth with your eyes turned skyward, for there you have been, and there you will always long to return." - Leonardo daVinci