Backup your data now.
What capacity out of the 64GB has already been written? Maybe you bought a fake one and the real capacity has been reached, so you get sudden failure. These fakes often appear larger, (e.g. 32GB instead of 64GB, or even 8GB instead of 64GB), then once the small capacity is filled, they lie to the user and overwrite the EXISTING DATA to appear like they still have capacity. Destroying your existing files (and showing a slow write speed as they have to do it in this slow, unorthadox way). This is not conclusive, so back up data and scan the card in a PC, preferably a dual-boot Windows-linux one with exFAT support, if exFAT is the filesystem used on the card. Then perhaps try formatting the card and testing it with proper tools.
Test the card: using automated tools, write to the whole card, read back the results as uncorrupted), sdcard.
Test with: https://fightflashfraud.wordpress.com/download-h2testw-free-and-test-flash-memory/
Note: even genuine flash can fail in time and has a write-cycle limitation.
Always backup data (in at least two other locations, plus your working copy on the card) ASAP.
Everything is expensive on an Indian salary, so if you can afford a camera, afford a decent, non-fake, TESTED card. With technology, I find that as long as the minimum speed is reached, reliability, not speed, is king!