I'm not sure if others have had exactly the same problem but this has happened a couple of times now with mine. I suspect that not using for varying periods perhaps corrupts some sort of EEPROM and/or whatever media the os is stored on. In any case, the solution seems to be to charge the unit for a couple of days ( even though it appears foobared ) and then attach the usb cable to a computer. Turn on the OP1 while holding the com button ( second down from the top rhs ) and you should then find the OP1 entering a bios screen. From here you can select an option to flash the firmware. You can download the latest firmware from the [https://www.teenageengineering.com/downloads/osupdates|TE website] and follow the instructions on the OP1 screen to do the firmware upgrade.
I'm pretty sure this second time with mine I already had the latest firmware so it isn't a matter of just upgrading the os, it's reflashing/formatting/writing it completely.
There was a useful [https://www.reddit.com/r/OP1users/comments/6p5xcj/wont_power_on/|Redit] thread about this too, as mentioned by Justin above.
It's a pretty nerve wracking experience having this happen, but there is every hope this will bring it back to life.