Did you drop or bump your drive? If so, then it may have a seized spindle motor or a stiction fault (heads stuck to platters).
Otherwise, it could be that your USB port can't provide enough current to spin up the drive. USB 2.0 ports are current limitd to 500mA whereas a 2.5" drive typically requires about 850mA to spin up. Some ports are OK with this, while others are not. In the latter case you can pick up power from two ports with a USB Y-cable.