More details and photos would be helpful.
It's not very common to see grain jars dry out to the point where growth stalls; have you done any shaking of the jar? How long has it been since it was inoculated? What was it inoculated with (agar wedge, LC, spores)? What's the temp? It could be contaminated and the contamination might only become apparent after a shake.
You can try injecting water if it's otherwise going to be tossed, but keep your expectations low.
For bulk substrates (trays) it's best to toss them and start over if there's a problem. The labor/risk/reward ratio isn't very good, so you'll put in more work than you'll get out in results trying to "fix" a sub vs. starting fresh. Material cost is generally negligible since straw and manure are cheap, so it's not worth it cost-wise either.