She is either retired or semi-retired and only sees old regulars who already reviewed her is the most likely explanation.

Contact the reviewers by PM if you are VIP and see if they have seen her recently and know whether or not she will see new clients or is still active.

How long? 2 months, 2 years?

Why not ask her directly?
Getting the information first hand is best. If she doesn't respond to emails and her phone is disconnected, that should tell you she is not available, reason irrelevant.

What should I take from the fact that a provider hasn't been reviewed in a long time?

She might have taken some time off from providing for various reasons.  She might have requested no more reviews.  She might be seeing only regulars that have already reviewed her.

or other national boards? Since TER doesn't allow me to name them, depending where you are located she may have more current reviews.  BTW, it's impossible to guess the reason, if you're already communicating with her you might  politely ask the reason for the gap in reviews. I have seen a number of escorts with long gaps in reviews, I've always had a great time, but I did my homework before seeing them.  If it really bothers you, just move on.

If she stopped running ads and updating her website back when her reviews stopped appearing, she's most likely out of the biz.