Problem with my old Sony Ericson K750i

Hi!
I am using Windows XP with my desktop computer. For the last few years upgrading and replacing my computer is not a problem with my cell phone. Eventually I have problems connecting with my cell phone. It keeps on saying “device not recognized.” What is the main cause of this problem? Is there any way that I can still salvage my cell phone and make it access with my computer? I know it’s an old phone but I haven’t have any problems in the last few years.
Please help me!
Thank you!
