OK, firstly do not panic. There is more than one path to qualifications but it may be a bit more wiggly than you, or she, expects.
She's been absent for more than 15 days. The LA are LEGALLY OBLIGED to provide her with a full time education. If school are not providing appropriate SEN support for her to access education, you can take action.
Depending on how good your LA is, send the template letter from IPSEA demanding provision.
At the same time, contact Early Help and ask for the dedicated link worker for your school. It's their role to make sure the school is putting in place all ordinarily available provision, and if you get a good one, they can be a lifeline in knowing next steps.
Also contact the safeguarding lead at the school. Find out what alternative provision they refer to, and ask them to make that referral on medical grounds - forcing your child into school without appropriate support is causing a deterioration in her mental health and she is at risk of ASD burnout at a critical time. Where are you in the EHCP process? If the needs assessment has been approved, that's a good indication that what is currently in place is insufficient for your daughter's needs.
Whilst all that is going ask, understand what GCSEs are needed for daughter's next step. If she's currently doing 9 or 10, what can she let slide? As she'll need to be self directed in the short-term, and no PDA child is going to manage a full time timetable doing that! Then keep up with the content of the main 5 or 6 GCSEs for what she's interested in at college. Ask the teachers to give you topics being studied this half term and any resources eg BBC bitesize, online quizzes etc. Don't set her work, but do "strew" it so she can easily find it when the mood grabs her. The worst thing I can say to my PDA daughter is "are you going to do your work?"!
Look on your council's Local Offer page. There will likely be charities who provide social groups. Contact them, join their Facebook groups, get on waitlists.