are you using EWF/HORM ?? It is ery unusual for the playlist to be empty since it is saved to disk as soon as you load it into the audio player screen... since it's a hibernation, both Winamp and RR should keep the last state of the playlist and require no additional processing.. Now, if you're using EWF/HORM you do need to make RR "load" the playlist again from disk...
1-Go into audio browser and Clear the playlist
2-Add a 5 songs to it (that you can easily identify)
3-Go back to the audio player/start playback
4-hit the power button
5-resume from hibernation (Playlist will show "empty" -- according to you)
6-Go back to the audio browser, highlight 1 song file and click +1 (ADD 1)
7-Return to the audio player and see if that file is the only one in the playlist or if the 5 songs you had (before hibernation) are there as well...
This will basically tell me if the problem is with Winamp or RR...