If you go guarantor on a bank loan, then you have signed a contract with the bank to the effect that you will take responsibility for the repayments if the recipient of the loan falls into arrears or cant pay. This is a big responsibility and bear in mind that a bank will only ask for a guarantor if they consider the recipient as high risk of not being able to repay the loan. So by going guarantor in the first place, you are fully aware that your friend is likely not to be able to pay at some point in time.
The bank does not have to exhaust all legal avenues i.e. take friend to court, before they pursue the guarantor. Once the friend is in arrears they are entitled to start calling in the guarantee that the guarantor has given.
From a legal perspective, as the friend is not or cannot pay, then this debt is effectively the OPs and the bank will probably take the usual steps that they take to recover any bad debt. If it is not recovered or recovered with difficulty, it will impact negatively on the OPs credit rating. A settlement with the bank to repay a lower amount and write of the rest will also impact on the OPs credit rating.
Its easy to say that in hindsight, the OP should not have gone guarantor if they could not handle the consequences of the friend defaulting. However, whats done is done and the OP needs to find the best resolution.
The solution will depend on the relationship with the friend and what type of character the friend is. If the friend is reasonable, then it could be along the lines of working out how much the friend can pay, maybe setting up a direct debit to ensure that they dont forget to pay and then paying the rest yourself somehow and reclaiming this from the friend at a later date or over a longer period of time. The fact that the OP says the friend has not paid any of the loan is not a good sign - indicates that the friend is totally irresponsible or has taken the OP for a patsy.