First I hope you have back-ups to this drive!!!!
If so, then the best option is to initialize the drive into a Mac format and transfer the data from your back-up onto the newly initialized drive.
WARNING: Re-formatting requires erasing all data on the drive, you will LOSE all data. Make sure your back up is complete and works.
Speaking of formats, how is this formatted? HFS+, NTFS, APFS???
Unless it is APFS with multiple "Containers" you cannot eliminate the partitions without erasing the drive. You could resize one partition to be very small and increase the size of the other partitions using Disk Utility, but you first need to move the data off of the partition so it has room to shrink. Again, if you erase or re-format, DU will erase all data when it reformats.
YOU HAVE BEEN WARNED!
If this drive indeed worked in the past, most likely something has caused corruption in the file system. If you have access to another computer, try plugging in your drive and see if it is recognized. If it is, then the corruption is on your computer and the data is intact. If it still is not recognized then most likely the drive itself might be having problems. Start by replacing the cables. Could be as simple as that.
If you don't have a back-up, you may have to start looking at data recovery apps or services.

Hope this Helps, Greg