Hello
Welcome to the HP Support Community.
This is a business class device and the battery can be removed kind of easily even though it is hidden behind a service cover. You can read the PC service guide >here< on page 65/75 to see instructions how to remove and reinsert the battery.
If the PC does not start even after that, make sure it is properly charged and follow the steps here
>> http://hp.dezide.com/ts/start.jsp?guide=NoPowerorBootIssues.net&as=true§ion=ccweb&sfs=tdoc&lc=en
Let me know how it goes - provide back any feedback/update/comments.