Mom put her house up for sale in Feb 2020, I have no POA. What can I do?

UK law as regards mental capacity where there is no Lasting Power of Attorney is quite complex. A decision that is in the person's best interests, and also complies with her own manifest wishes from when she was of sound mind, and that the lady appears at the time of signing to understand and be content with, probably wouldn't actually raise any eyebrows.

Assessment of mental capacity has to be specific to the decision in question - so you can't just catch her on a bad day and decree that she can never again make a major decision, for example.

My first step would be the OPG; I would also expect the solicitor to have a colleague with the right specialist experience to assist with this. Certainly don't despair - it may be a pain, but there will be a way round.

Who at the home is saying that your mother does not have mental capacity? - and what else is the home saying about it, e.g. are they talking about appointing an advocate, suggesting you apply to the OPG, or what?

Your mother apparently sold the house before entering a care facility. It seems to me that she had the clarity of mind to take that action, perhaps anticipating that she wouldn't be returning.

I'm assuming you're in the UK since you used the word solicitor; have you contacted him or her to discuss options? It may even be that she did execute a limited POA for sale of the house and left it in trust with him or her.

I don't know anything about UK law, so I won't try to offer any other suggestions but I do think that contacting the solicitor is your first step.

Who exactly at the nursing home told you she lacks capacity? How was it determined? Just by observation, or by a medical professional?

When we sold my mom's condo, she was clearly losing it, but not formally diagnosed by a DR.   Just because nursing home says she is not competent is not the same as DR.  We got a notary to go to her home and he notarized POA and we sold condo.  Now, on her good days, she could appear coherent AND all children were in agreement as to what to do. 

In the era of Corona, this is not likely possible.  Agree with pp, in the era of Corona, you may have to consult lawyer and get guardianship.  ugg.

Who is leading the house sale? If the NH says she doesn't have mental capacity for decisions, how did she find a realtor? I agree with Gladimhere about consulting an elder law attorney but if your mom won't assign you as a durable PoA then you will need to pursue guardianship or the county will be doing this. Someone will eventually have guardianship, even if it's not you. If I were in your shoes I'd have her mental incapacity diagnosed by a doctor first so that this may slow down or stop the sale of her house. Then I'd find out who is behind this sale and let them know her diagnosis and that if they attempt to move it forward they are in danger of elder financial abuse/fraud. But get the diagnosis in her records first. If she is in AL and is resistant to seeing a doctor, it is acceptable to tell her a "therapeutic fib" like, Medicare or SS now requires it to receive benefits. Good luck!

Call an elder law attorney for help to see what can be done.