Hi my 11 week old has started to roll on her side during sleep. She sleeps in her cot now and I’m concerned because she hasn’t learnt how to roll back. How do I stop her from rolling onto her face. What should I do ?

Most babies will be rolling over by 4- 5 months. Some earlier, some later.
This is very normal as babies grow & become stronger.
Many babies then prefer to sleep on their side or tummy.
This is not something that needs to be stopped. It is important to make her new environment as safe as possible.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

As you have done, it would be safest to also transfer her to her cot now.
Are you still wrapping/swaddling her? Important to stop now she is starting to roll.
Red Nose recommends - space of the cot & arms free, & firm, flat mattress to provide safe environment once rolling.
Remember, still always place her on her back for sleep.

What happens if you try to roll her back when she is on her side or tun her face to the side?

How does she go with tummy time when awake?
Increase her tummy time when awake - more frequently if she is not tolerating long periods on her tummy.
Increased tummy time when awake helps to build upper body strength to assist with rolling both ways.
Continue with nothing loose, soft or padded in her cot.