I have a 64st which I usually store without the 2 AA batteries (in case the batteries leak). Today, after leaving it for maybe two months I put fresh batteries in and tried to turn it on. No response. So I did an internet search and came across this , below, on a 2017/8 discussion thread on gps-forumsdotcom . What happens is the internal battery becomes discharged. This procedure described worked for me as far as re-activating the internal battery so that the unit can be turned on by the switch. I'll leave my batteries in now.

“If you store your unit for a long period (two months) without batteries it may enter in a failure state.
In such failure state, the unit refuses to start up when you put the batteries back.
Once in failure state, no matter how long the batteries are back in the unit, it won't start up.
A workaround for this situation is plug the unit in via USB connection. You can connect the unit to a computer or even a USB power bank. After only some seconds connected, the cable can be removed and the unit is expected to start up normally.
However, if you remove the batteries after a couple of hours, the unit may enter the failure state again. To avoid this situation, be sure the workaround above was successfully executed, power off the unit and then store it with the batteries in for several days. After this "recovery period" the batteries may be removed and the unit will survive for a extended period without entering the failure state.”