So, it looks like this model of laptop has an emergency partition for instances such as this. Basically, you'll boot up in recovery mode using this partition instead of your primary partition (where your OS lives)
This link is from a previous discussion I found here on Ifixit. Give the first answer a read, he gives good instructions, and well as video links that show the process step by step.
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And for future reference, older machines will usually have all questions answered for this kind of stuff just with a quick search. Sometimes that'll be quicker than posting to a forum.
Hope this helps!