Answer
Months out of sight can be normal, especially for fossorial species, slow-growing adults and spiders in a long premoult.
Leave a sealed retreat alone
If the burrow is stable and the enclosure is not flooded or overheating, do not dig the spider out simply to check it.
Water
Keep the water dish available and maintain the normal species moisture pattern. Do not pour water directly into a sealed burrow.
Feeding
Do not push live prey into a sealed retreat. If the spider emerges and the entrance is open, normal feeding can resume when there are no post-moult signs.
When to investigate
Act if the enclosure itself becomes unsafe: flooding, collapse, extreme temperature, severe mould/contamination or another clear physical hazard.