Is the printer paper jammed?
If so, gently pull out some paper near the closure and print a test receipt to see if the issue is resolved. If successful, you should be able to print a new receipt.
Is the text on the receipt faint or hard to read?
If this is the case, open the printer, remove the receipt roll, and clean the printer body. You can do this by blowing dust out of it and wiping it down. After cleaning, print a few test receipts to see if the print quality improves.
If there's no improvement, try printing several receipts to warm up the printer, as it's a thermal printer.