After an ex-girlfriend demands a sperm sample in exchange for giving back a rare vintage guitar, a once-famous rock star and his landlord’s daughter come up with a plan to get the guitar back without giving in to his ex’s demand while, also, dealing with their growing attraction to each other.

When a deranged groupie attempts to blackmails him into fathering a baby in exchange for his rare vintage guitar, a reformed former hell raising rock star recruits his landlord's daughter to steal it back so he can pay his debt in time to a violent loan shark.

Still way to long, but at least now there is a motive 'repay his debt', a time limit 'in time', a threat in case of failure 'violent loan shark' and an antagonist 'deranged groupie'.

I guess the most obvious question is, why not just give her the sperm sample?

This is too long and unclear. Here are all the questions that come to mind, no need to answer them here just take them into account when rewriting the logline.

What is the worst that would happen should he not get the guitar?? Doesn't seem like much so the stakes are really low.

Why a sperm sample? This is a strange thing to demand, it isn't clear whether she wants him to under go a paternity test or is obsessed with him in the weirdest of ways.

What has the landlord's daughter go to do with any of this other than being an ally? You can cut her and the statement about the growing attraction out.

What is his flaw? Being a once-famous rock star isn't something a character wants or needs to overcome.