Whether or not you should cancel the Sprint and start over depends on whether or not your lack of ability to finish those stories translates to a lack of ability to accomplish your Sprint Goal.
At the beginning of every Sprint, the Product Owner must define a Goal for that Sprint. It should generally not be simply "finish stories 1 and 2 and 5 and 6 and 7 and 8".
If the Sprint Goal becomes untenable, then yes, you should abort the current Sprint and plan a new one (note that it should be the Product Owner to make this call). If it's still possible to achieve the Sprint Goal, then it doesn't really matter that Stories 2 and 6 aren't going to be done. Just re-estimate them next Sprint (possibly choosing to defer to a later Sprint or remove them outright).
Afterwards, regardless of whether or not the PO decides to terminate the Sprint, the situation should be brought up for discussion in the Sprint Retrospective meeting. If this is a recurring issue, then you need to look into changing your expected velocity. If a one-time issue caused by extraneous events, it should be determined what (if any) measures should be taken to mitigate that same risk in the future.