First, make sure you have the photos on the computer. Once you know they are - then you know you have a working copy of them.
The problem you're describing can be the result of MOVING the files - rather than COPYING the files. Moving is the process of moving the original files from the card and putting them on the PC - while copying leaves the original files on the card and creates a copy on the PC. If you MOVED them then you shouldn't expect to see the original files on the card any longer.
If you copied them and the card no longer shows them or if you copied or moved them and the card is no longer visible the card may have been removed before it completed the file move / copy operation. You should only remove a card / usb drive / similar device when a message pops up saying it is safe to do so.
If: 1) there are no files visible, 2) the card can no longer be read or 3) your computer says the card needs to be formatted first - you may need to do that in an attempt to make the card useable again. If you do not have a copy of the files on your PC - try to copy them to another PC before formatting the card. If you can not get copies of the pictures / files on a 2nd PC, the files / pictures on the card are likely lost. You may find utilities than can restore them however (see link below)
recover pictures from memory card Google Search
Good memory card management includes 1) copying files from memory cards, 2) verifying that they have transferred successfully *and* finally 3) formatting the memory card *in* the camera; *not* the PC. Most cameras offer options to format cards - which is superior to simply deleting files or formatting in a PC. The camera tends to set the card up for its own use - rather than for a PC's use. Formatting the card in the camera can introduce subtle differences that make it worthwhile to do.
Not as much a big deal nowadays - but older devices, cameras, etc. can read / write only to "straight" SD cards. Today, there are SDHC and SDXC types. Make *sure* that you're using the type card your camera expects. Using an SDHC or SDXC card can have unpredictable results when used in devices that expect the older, "straight" SD (no "HC" and no "XC" suffix) type cards. More details about card types can be found linked below:
Back to Basics The Difference Between SD SDHC SDXC and Which is Best for...