These days happen. For me the most consistently associated factor was how hard/long I played the day before. I found that a longer warm up can help. I also revise my expectations way down when it happens. Shorter session, nothing really difficult, less work in general, more breaks, longer breaks, no particular attachment to outcomes. If possible, several short sessions through the day. If preparing a solo piece that's long, I'll take plenty of breaks. Concentrate on phrases in the piece, short ones, with breaks in between. Do Clark studies at low volume. Just make some sound and don't worry to much about anything really. Sometimes it actually gets better to the point of almost a normal day. Be careful not to overdo it, however, less the following day be even worse.