Probably has internal batteries (often CR2032's), which have died. You might as well try replacing them - you have nothing to lose. Worked for my Murex helmet.
Take the panel out and remove any lenses. Lay flat and cut round the join on each side with a Stanley knife (other brands etc!) . When separated, cut out the batteries. solder some thin wires to the original battery connections and lead out through the top edge of the panel. Superglue the panel back together. Get some new batteries and battery holders (get mine from Maplins). Solder wires to the holders and attach the holders to the inside of the helmet - I use a good blob of silicone sealant. To make it easier to remove the panel and replace the lenses as necessary, it will make life easier if you put some Tamiya battery connectors (also from maplins) in the wires. You have a choice of using AAA, AA or CR2032 batteries.
Had an esab eye/tech fail on me a couple of years ago,
tried disecting it to replace the batteries but it seemed virtually impossible to get open,
Gave the helmet to a mate as spares for the same helmet he has,
not bitter as it was a good ten years old and about 5 of those was i was a full time welder/fabricator.
Just one question about replacing the batteries; if they are CR2032 then most off-the-shelf CR2032 are not rechargeable, and I assume that auto-darkening helmets do recharge the batteries, right?
On reading this thread I did a quick google and it seems that you can get rechargeable li-on CR2032 but that most off-the-shelf versions definitely aren't rechargeable.