Hello,
Welcome to toddler sleep
Sounds like this started at around 18 months and there is an 'infamous' sleep regression then which may have kicked off your problems.
We also went through this regression but once it was over the good news is the re-sleep training didn't take that long.
I would suggest that you start settling her again at BT in her own bed (am I right you are still doing this downstairs ATM?). She will prob protest at this age but you have to be consistent and teach her again that she goes to sleep in her bed. At first, I'd suggest staying with her and giving her as much reassurance as she needs to fall asleep.
Then you could use GW to gradually reduce the amount of help you give her until she is sleeping independently again. Alternatively, depending on how you think she'd react and since she used to fall asleep independently, you could use WIWO.
This link describes these techniques in more detail:
Walk In/Walk Out vs. The Gradual Withdrawal Method (HOW TO CHOOSE)What do you think?
It will prob be tough for a while but I think you really need to teach her to fall asleep independently again.
Also, I wondered is there any teething going on at the moment as canines can mess up sleep a lot.
Could you also post her usual routine (WU, nap time and BT) as maybe a tweak could help too.