Mic not as loud as it used to be 1 month ago

Guys, please help me! i have basic m-audio m-track and shure sm58. at first i had preamp gain at almost 100%, but it sounded great and loud. now, 2 months after, with the exact same options, my mic is about 5 times quieter. can it be m-track problem and how do i fix it?
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