Tough one – stuck mashes suck! Underletting will lift the bed unblocking it. You can then recirc slowly and try again, run off real slow. You can also manually make channels in the grain bed but wort clarity will be poor.
They can be caused by a few things, an iodine test will confirm correct conversion. Black and there is starch present. Check all the mashing conditions and double check temperature probes. No colour change and it’s a grist composition, loading or milling issue.
Check out Carl’s blog on mashing.