Will anything terrible happen if I miss a dose of alavert?

No. You should not have much of a problem missing just one dose of alavert. It is a long acting antihistamine, so likely the most you might notice is an increase in allergy symptoms.
No. Nothing serious can happen if you miss a dose of alavert (loratadine). Typically missing a dose of antihistamine triggers symptoms of itching, itchy rash, sneezing, itchy eyes, and increase in nasal drainage. These symptoms can be relieved once the medication is restarted.