@Wh0-Am-1 bro i am too facing this same problem...
as of now you simply cant retrieve your lost tabs and nobody here can help you ,you have to be prepared next time by use of session buddy or other 3rd party extensions.
here real developers will never heard our problems as some shitty moderators keeps on banning people for simply asking question in both opera+opera gx browser twice(you people are so active in noticing this type of errors but will never understand what a user is going through ..why not you help instead or dm personally to remove /modify the new thread)

In Short -i simply closes the stock 1st workspace first( w/o enabing workspace to enabled)and then the temporary one,when opened again "retain my last session" kicked in and i lost all my alot of imp tabs.
-I had near about 120 opened tabs and by mistake 1 tab came out hurridly (from my mouse pad as i am in pc:one fresh tab)and it acted like 2nd workspace
[i didnt know about this till i read about it here in other forums and never installed session buddy nor taken any precaution to save sessions regurarly] && never enabled dual workspace /environment something from setting.
Now my mistake is i closed the earlier workspace with many tabs open first then after a while closes the 2nd workspace which have nothing useful .
I didnt restart my pc hoping i can somehow get it back from some cache/backup files/from cloud as i had turned on my sync from start.
//i had referred to earlier posts where many user already faced similar issue but coming here in inly hope that in recent version LVL2 (core: 71.0.3770.323) ,it maybe possible and some techie out there can help me and with all due respect sir...plz help me out soon or else i fear i may lose all my tabs once i lose the cache or session..whatever//