The thermal event could have been the CPU overheated. That could be due to several possibilities. (1) Dust inside that is clogging the CPU heatsink, cooling fan, etc (2) PC located where it cannot get adequate (cool) air flow (3) heat sink compound between the top of the CPU and the bottom of the heatsink is bad and needs replaced (and/or the heatsink has come loose and not making contact, which will also require replacement of the heatsink compound). These are the most logical, especially considering the high temperatures in many parts of the US.
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Dell forum member since 2002
Dell Inspiron 15 - 5577 Laptop
Home Built Desktop PC with ASUS ROG Z170, i7 6700K CPU
Windows 10 64 bit Pro and Windows 10 Insider Program Beta Versions. SSD drives. Cakewalk by BandLab and Studio One 4.1 Recordng Studio Software.