The start codon is responsible for the initiation of translation in both prokaryotes and eukaryotes. AUG is the start codon in both which code for methionine in eukaryotes and formyl methionine and prokaryotes.
Therefore if the starts codon becomes mutated than the tRNA would not able to recognize the initiation codon and no methionine would be added as the first amino acid so the translation process might not start and no protein forms. Then the ribosome scans further for the next start codon to start translation.
