It depends on how hypo she is and whether or not she has hashimoto's (autoimmune). 6 months is long enough not to have any medicine left in her system whatsoever so she is probably as hypo now as she's going to get without it (unless it's autoimmune in which case things could go worse for her).
Likely fatigue, weight gain, water retention (myxedema), hair loss, vision loss, muscle mass loss, heart problems, well, everything really... leading finally to a coma. This is what could happen to her... her thyroid could be taxed to the point where it starts swelling up to the size of a balloon and flops over her neck... or she could just feel sick and tired for the rest of her life until she dies of heart failure at an early age. It depends on her personal sickness.
Thyroid disease is easily treatable with the proper medication and the medication is not really that expensive! I pay $20/month for a prescription plan and on that plan I pay $8 for my medicine every month. There's no way your mom can't afford that! She let her prescription lapse and that's the problem, now she'll need to go back in and see a dr. and get blood run.
Have her try to check into a low income health clinic for testing... or sign up for state health ins. which is based on income... anything... she needs help with this. It's a serious health risk.
Mom's Hypo and has not taken medicine in 6 mos! 
