Agree with John on both points: your post is very low on details, and trying another card is a good first step.
Some things that may help clarify the situation:
1. Have you used this specific SD card before?
2. Has it previously had any issues?
3. Is it a full-size SD card, or a micro SD card with an adapter?
4. Have you checked the card lock is not set to on?
5. Have you formatted the SD card previously, or just deleted files?
If you have another full-size, SD card, insert that and see if the camera reads it. If it does, then the card is almost certainly the issue, if not you may have some other issue with the camera, but we'll deal with that when we know.
With the card that is not working: after making sure the car is not write-protected, put the SD card into your computer and format it there. If it reads the card, download the images to the computer and format the card, then try the card back in the camera again. If not, it is likely that the card is corrupted - replace it.
a. You should have several full-size SD cards. After each shoot, you should download your images from your SD cards to your computer and then format them (preferably in the camera, but on a computer will do). Just deleting images causes small errors on the card that can accumulate and turn into big errors.
b. If you remove a card from a computer you should go through an EJECT process - on a Windows system, within File Manager (or My Computer), on the left, RIGHT-CLICK on the card and select EJECT from the drop-down. menu. If it will not eject, then a program or process is using it, and just yanking it out can permanently corrupt the card. I don't know the process for a Mac.
c. Always avoid Micro-SD cards with adapters, they do not work well on many cameras (and that's not limited to Canon). If you get the full-size cards, beware of extremely cheap cards - there are lots of low quality cards from masquerading as good units. You get what you pay for.