Samsung YP-Z5F
I've had various issues similar to those described, and can offer the following trouble-shooting and/or work-around tricks that I've had success with:
Sometimes, when I come to use it I find that it won't turn on at all, but it's most likely because it's been a while (several weeks) since the last time that I used it. The battery does seem to lose its charge over this time. All I do is to charge it up for several hours, (or even overnight). It usually charges best by leaving the unit OFF when plugging in the USB cable to the PC, just like a cell phone does.
As for the problem with the PC recognizing it, sometimes it just takes a few minutes. I have winXP.
First, open MyComputer. If the mp3 player doesn't show up as another drive after about 60 seconds, press the F5 key to 'refresh'. Sometimes closing MyComputer, then re-opening it makes a difference, too. You might also want to try a different USB connection, like using one in the back of the PC instead of the front (or vice-versa).
Sometimes, though, it doesn't hurt to just press the RESET button at the bottom side (6 o'clock position when holding it upright and facing you). With it turned ON, I insert a paper clip that's been partially straightened out, holding it in for about 3 seconds.
Good luck!