I suspect your UDP symbol contains one or more objects that are a long way away from the objects that you think make up the symbol. This would explain why you have the red selection handles but cannot see anything: to fit the whole symbol into the window the zoom is so far out that you cannot see individual objects.
Feel free to email a project file that shows the problem to
techsupport@stardraw.com and we will be happy to investigate further.
You might find it useful to watch the movie "Blank Imported DWG" at the bottom of
https://www.stardraw.com/sd7/movies Assuming my suspicion is correct you will be able to use the same technique to fix the UDP symbol by editing it: in the Product Browser, rightclick on the UDP and choose Edit Symbols, then use the technique described in the movie to locate and remove any displaced objects.