Be sure to test the phone after performing each of the given procedures to see if the problem is fixed. Otherwise, proceed to the next step.
1. Ensure you are using a memory card that is supported by your device. You can verify this information in the user manual.
2. Power off the phone.
3. Wait for about 5 seconds and turn the phone back on.
4. Attempt to access the files on the memory card again.
5. If problem persists, test the memory card in another device available. You can use a different mobile phone, digital camera on other devices with memory card support.
6. If the memory card is not recognized by other devices, then it's a memory card problem. In this case, I suggest you try to reformat the memory card (preferably using the FAT32 format) and try again to see if this fixes the issue.
If the memory card can't be read after it's been reformatted, then your option is likely to replace it with a new one.
If the memory card is recognized by other devices, then the problem is most likely on your phone. I suggest you contact your phone carrier to let them know the issue exist after the Lollipop update. Once the issue is escalated, they should be able to assist you further using their advanced troubleshooting tools.
Or if you want, you can try to perform a hard reset on your phone. Some users who have encountered the same problem with the S5 devices after the software update have found fix by performing a hard reset.
But before you do this, please back up all your important files to a different device for easy recovery afterwards. Please note that all your personal information will be completely deleted after the process.
Here's how to perform a hard reset on the Galaxy S5, in case you need help in doing so later.