When my cat was a kitten, she had a litter box. Due to bad communications between us who lived there, the box did not get cleaned as often as it should, so the cat started doing her business only outside. We kept the box around for some time, but she never used it again, and we trashed it.
Some years later, the cat disappeared. She was found six weeks later 20 km from home, and returned to me, thanks to her chip. After this incident, she was afraid to go outside. She would go out if I accompanied her, but was reluctant to go alone. Naturally, the cat still had to do her business, so carpets, blankets and corners became targets for her needs.
We bought her a new litter box, and gave her time. She gradually started to go more and more outside alone, but actually came in to use the litter box when she had to go, then jumped out again. Now, a year later, she is back to her normal self, and hasn't used the box in months.
So my suggestion is to get a litter box, and teach your cat to use it. DO NOT shame your cat for going on your furniture if it has no alternative. See Sienna Vaile's answer. Then just give it time. Maybe it will fall back into its old habits, maybe it won't.