Hi and welcome!
I am so sorry for your loss! Sadly, guinea pigs can die at all ages seemingly out of the blue from heart attacks, sudden acute heart failure or strokes. You are welcome to post a tribute to Pebbles in this section if or whenever it feels right for you.
What is Pebbles' companion called and how old is she?
This detailed guide here gives you all the tips of what you can do now, what to look out for in the ext few days and what your options are for the longer term:
Looking After A Bereaved Guinea Pig
Right now, Pebble's companion is most likely grieving and withdrawn. Please monitor her food intake (weigh daily before you feed her dinner) and if necessary, step in with some syringe feeding (mushed pellets will do). I have made the experience that a bit of syringe feed can stimulate the appetite and encourage a pining guinea pig to start eating by itself again. As long as a bereaved guinea pig is eating by itself, it is not acutely pining and still has the will to live.
Complete Syringe Feeding Guide
Your longer term options very much depend on where you are located. We have got members from all over the world. You can help us to discuss the pros and cons of your relevant options with you in more detail if you please added your country, state/province or UK county to your details; this enables us to always tailor any advice and recommendations straight away to what is available and doable where you are. Click on your username on the top, then go to personal details and scroll down to location. Thank you!