You are unable to turn up for your placement at all. In this scenario you never arrive at all, due to some unforeseen or personal circumstance.
A natural disaster, like a flood, or the outbreak of war, or perhaps as a strike by the staff of your host institution, would all count as force majeure. In those circumstances you would not have to pay money already spent for the period you were at the institution. However you will have to return the part of the grant already received for any unused period.
Minor illnesses will be covered by your European Health Insurance Card, but you will be required to have insurance to protect you in the case of serious illness.
In principle it is often possible to extend your stay. You may have originally come for one semester and now want to finish the year, for example. The maximum stay possible is for 12 months, except for some single-cycle programmes, like medicine, where 24 months is possible. The extension period cannot be longer than the original grant period was.