。
The button battery for the PC's internal timer has died.
Literal
PC-inside [genitive-の] timer-use button battery [subject-が] cut.
Tech writing. 切れる ('to be cut, run out, expire') is a versatile intransitive verb used for batteries dying, contracts expiring, stocks running out, etc. 用 as a suffix ('for the purpose of') attaches to nouns: タイマー用 = 'for the timer, timer-use.' This ~用 pattern is common in product descriptions: 事務用 (office use), 家庭用 (home use).